1. Technical Field
The present invention relates to a paper drying machine and a method of drying a paper web in a paper drying machine. More particularly, the invention pertains to a paper drying machine which employs through air drying of a paper web in two different directions.
2. Background Art
Paper products have conventionally been manufactured by forming a wet paper web on a fabric carrying sheet which then carries the paper web through a pressing section to remove the excess water from the web. After pressing the web to remove excess water, the paper web would then be fed to a separate drying section to fully remove the remaining moisture from the web. This step of pressing the web, however, did not function effectively with the presently required high-speed operation of such papermaking machines. Therefore, as opposed to leaving the web as a flat sheet on a single plane, rotatable steam-heated drum dryers over which the web traveled were utilized. The cost of supplying steam to these multiple drum dryers for heating the dryers is quite high and the ability to maintain uniform drum surface temperatures and humidity in the dryers is difficult. Accordingly, dryer hoods are widely used in connection with these rotary drums, wherein pressurized drying air, instead of steam, is introduced at various points in the hood to contact one exposed surface of the wet web as it progresses around the dryer with the exit path for the air being positioned on the other side of the rotary drum. This process is known as through air drying (TAD).
U.S. Pat. No. 3,432,936 issued to Sisson discloses one such drying assembly in which a moving stream of pressurized drying air is circulated about a paper web traveling about the periphery of a rotatable roll having apertures formed therein. Sisson utilizes a system where the hot drying air travels from the inside of the rotatable roll to the outside through the apertures, while the web travels about the outer surface of the roll. With such rotatable rolls usually being composed of metal, this inside-to-outside type drying requires smaller diameter rolls because the roll metal is on the hot side of the web, and larger diameter rolls create roll integrity problems with this inside-to-outside drying.
U.S. Pat. No. 3,432,936 issued to Cole et at. avoids the problems with inside-to-outside drying by employing a configuration which moves drying air from the exterior of a rotatable roll through the paper web and into the interior of the rotatable roll, otherwise known as outside-to-inside drying. This configuration positions the metal rotatable roll on the cool side of the paper web which allows larger diameter rotatable rolls to be used. However, when two or more of these rotatable rolls employing outside-to-inside drying are used, at least two carrying rolls must contact the paper web. As discussed earlier, whenever the paper web contacts a carrying roll machine runability problems as well as product quality problems are encountered.
One of the most important shortcomings associated with the above-described paper drying machine is that the paper web must come into contact with a carrying roll whenever more than one rotatable drying roll is employed. Further, machines which have one roll have limited drying capacity and are of limited commercial interest for that reason. Moreover, prior art machines employing multiple drying rolls have the drying air traveling in the same direction in each of the drying rolls used in such a machine. However, the most efficient use of space in these machines would be to use a combination of inside-to-outside drying rolls with outside-to-inside drying rolls. U.S. Pat. No. 1,718,573 issued to Millspaugh discloses a paper making machine which discloses removing moisture from a paper web in an outside-to-inside fashion using a suction roll followed by an inside-to-outside removal of moisture by forcing steam through the paper web as it passes over a blower roll. It should be noted, however, that the device disclosed by Millspaugh utilizes steam with its blower roll, wherein the same problems discussed in the above prior art associated with using steam in a paper drying device are present in Millspaugh. Furthermore, it is economically more efficient and proficient to employ heated air rather than a combination of air and steam when using a blowing device to dry a wet paper web.
Therefore, as can be seen from the foregoing, there is clearly a pressing need for a paper drying machine which utilizes through-air-drying rolls of the above mentioned type, wherein the drying rolls employ two different directions of drying in order to make optimal use of space in the drying machine. Furthermore, there is a pressing need for a paper drying machine having no carrying rolls contacting the paper web, thus reducing any potential runability problems with the roll dryers.